A method of one-stage catalytic oxidation of sulphur dioxide in non-stationary condition is known, where the direction of the incoming reaction mixture, is periodically changed (U.K. Patent No. 208521 (Apr. 28, 1982); USSR Patent No. 994,400 (Feb. 7, 1983); DAS No. 3,050,368 (Jun. 3, 1982); U.S. Pat. No. 4,478,808 (Dec. 24, 1981)).
The comparatively low degree of conversion (90-96%) at sulphur dioxide concentrations above 3 vol. % is a disadvantage of the method.
A method is known for the production of SO.sub.3 by a two-stage catalytic oxidation of SO.sub.2 and an intermediate absorption of SO.sub.3 after the first stage of oxidation, the so called DC/DA (Double Conversion/Double Absorption) method (Bulg. Patent No. 39,528 (Mar. 5, 1985)).
In compliance with the method the first stage of oxidation represents a conventional three-layer reactor of intermediate cooling of the gas flow between the layers, where 90-93% of the SO.sub.2 is oxidized to SO.sub.3. After cooling of the gas flow, coming out of the first stage, to 150.degree.-180.degree. C. and an intermediate absorption of the obtained SO.sub.3, the gas flow of the concentration of 0.7-1.5 vol. % SO.sub.2 at the temperature of 50.degree.-70.degree. C. enters the second stage of oxidation. The latter represents a reactor of a stationary layer of catalyst preliminary heated to 450.degree. C., operating in non-stationary conditions that are achieved by periodic changes of the direction of the incoming gas. The obtained SO.sub.3 is absorbed in an end absorber.
The first stage of oxidation works in an autothermal regime, i.e. the heat of reaction of the system should be adequate for preheating the incoming gas flow from 50.degree.-70.degree. C. to 400.degree.-450.degree. C. at the entrance of the first catalyst layer of the reactor.
A disadvantage of the above method for SO.sub.3 production is that the autothermal regime of operation of the first stage of oxidation is disturbed when the flow rate or the concentration of SO.sub.2 in the input gas flow is changed, which gives either insufficient or surplus heat in the system at increase or decrease of SO.sub.2 concentration, respectively, which on the other hand requires a substantial reserve of heat-exchange surface.
In another known method for the production of SO.sub.3 by a two-stage catalytic oxidation of SO.sub.2 the two stages operate in non-stationary conditions achieved by periodic changes of the direction of feeding the input gas flow (Bulg. patent Registration No. 79,942 (May. 29, 1987)). In this method the catalyst layer in the reactor is separated into two equal parts, where after the first layer in which the gas flow enters the reactor flow is cooled in an outer heat-exchanger down to 130.degree.-160.degree. C. and an intermediate absorption of the obtained SO.sub.3 in the first stage, takes place. Then the gas flow is preheated up to 250.degree.-350.degree. C. and goes to the second layer of the reactor for after-oxidation of SO.sub.2. The obtained SO.sub.3 is absorbed in an end absorber.
The disadvantage of this method is that when SO.sub.2 concentration decreases to 4 vol. %, it is necessary to increase the inlet temperature in the second stage to 350.degree. C., which is done by partial bypassing of the hot gas flow at the outlet of the first stage of oxidation directly to the second stage, i.e. intermediate absorption. That lowers the overall degree of conversion.
When SO.sub.2 concentration is above 8 vol. %, there is a heat surplus in the system, due to which the maximum temperature in the reactor increases and the overall conversion decreases. On the other hand, when the direction of entering of the gas flow to the reactor is changed, the hot and the cold gas flows in the intermediate outer heat-exchanger change their places, i.e. a part of SO.sub.3 will not be absorbed, which again lowers the degree of conversion. In the same time, the intermediate absorber should operate in an alternating regime of movement of the gas flow related to sprinkled acid or must have its own system of valves for changing the direction of gas feeding to it, operating in synchronization with that of the reactor.
A common disadvantage of the described methods, as well as of classical DC/DA schemes, is the disturbance of the temperature regime of the system when the flow rate and the concentration of SO.sub.2 in the gas flow are changed. Besides, it is not always possible to realize such a degree of conversion, which can guarantee a concentration of less than 0.05 vol. % of SO.sub.2 at the outlet of the system.
It is the aim of the present invention to overcome the above mentioned disadvantages and the provide a method for the production SO.sub.3 by a two-stage catalytic oxidation of SO.sub.2, contained in an inlet gas flow in a contact apparatus (reactor), operating in a non-stationary regime, which is realized by periodic changes of the direction of the gas flow to the reactor and intermediate absorption of SO.sub.3, providing a stable regime of operation of wide limits of variation of the flow rate and the concentration of SO.sub.2 in the input gas flow as well as lower than 0.05 vol. % SO.sub.2 concentration at the outlet of the system.